In contrast to "normal" connections, multichannel connections have very special synchronization requirements. These requirements can be described as follows, for example with the usual time-division multiplex transmission in telephone networks (32 channels, time frames of 125 .mu.s):
1. The sequence of the channels within the frame must be safeguarded. PA1 2. The channels assigned to a multichannel connection must be time-synchronized with each other, i.e. the PCM (Pulse Code Modulation) words of all channels of the multichannel connections must belong to a single frame of the transmitter as well as of the receiver.
When the second request for synchronization is fulfilled, there are problems because the transit times of different paths through a switching network do not generally coincide. These deviations are the result of transforming channels to the time-stages of the switching network.
The invention now starts with a known solution for the fulfillment of these synchronization requests, as described for example in the European patent EP 85 110 673.2.
The channels of a multichannel connection are conducted like single channels through the switching network of an exchange. A buffer is provided behind the switching network, whereby the channels of the multichannel connection are synchronized. The delay of individual channels required for the synchronization is determined by the transmission of frame numbers in the individual channels.
This solution has the disadvantage that a large output-side buffer could be required under certain circumstances.
Further solutions are known from the American patent U.S. Pat. No. 5,136,578:
A further solution is to synchronize the channels of a multichannel connection during each switching network stage. To that end an additional frame is temporarily stored in each time-stage. This solution has the disadvantage that the transit time through the switching network becomes approximately double.
A further solution includes providing special assignment specifications between the input-side and the output-side channels in each time-stage of a switching network, and in this way possibly avoid a time displacement between different channels being switched through the network. Although this proposed solution reduces the required synchronization buffer, the path selection within the switching element is restricted and special switching elements are required.